Measure the length and width of your wicker trunk. Purchase a chair cushion that is the same size as the top of your trunk. These can be found in most large retail stores. Select a color that will look good in the room and with the wicker finish.
It may be possible to get a tie-on style of cushion to fit your ottoman. If the ties aren't too thick, they can be pushed through the wicker and tied inside the lid. Otherwise, get two pairs of flat bootlaces (such as a nylon weave). Or get 4 feet of 1/4-inch flat trim from a fabric store.
Fold the shoelaces in half, or cut the trim in 12-inch lengths and fold those in half. Sew four of these ties to the underside of the cushion, 2 to 3 inches in from each corner. Center the cushion on the wicker trunk lid, and push the ends through the wicker at least 2 inches apart. Tie the ends together in each corner. This will secure the cushion.